Thin-film solar cell technology based on appropriate semiconductive materials, such as amorphous silicon (a-Si:H), is very promising for generating low-cost solar energy. This thin-film solar cell technology may be used for cost-effective applications such as large area photovoltaic modules and cells applied to any appropriate carrier material. For example, semiconductor materials and metallization layers may be applied to flexible substrates, thereby enabling the production of lightweight rollable and/or foldable solar modules which allow efficient storage and transport.
Generally, the particular size, shape and design of thin-film solar cells formed on flexible substrates allow for innovative design of circuit interconnections including efficient designs that may minimize the impact of shading effects that would otherwise be impractical for conventional large area solar cells.
In particular, photovoltaic flexible modules are highly advantageous for indoor energy harvesting applications. In this case, the solar module has to comply with the requirements of low intensity of light that is available from indoor lamps, such as fluorescence lamps. The intensity of such light sources is about 1/1000 of the sun light intensity under standard outdoor conditions (radiation travelling a distance through the atmosphere that is 1.5 times the height the atmosphere). In addition the spectral composition of radiation emitted from indoor light sources is very different from the spectral composition under outdoor conditions. Under these specific environmental conditions the performance of the solar cell is typically reduced due to a significant impact of defects and parasitic resistances of the solar cells. In particular the impact of variable dark leakage current at low biases, which is commonly referred to as shunt leakage current, is noticeable at reduced light intensities. When this shunt leakage is sufficiently high it reduces the fill factor, i.e. the ratio of the maximum power point (MPP) and the power defined by open voltage and short circuit current of a solar cell, thereby adversely affecting the cell efficiency. Hence, for solar modules to be operated mainly under environmental conditions with reduced light intensity, for instance in indoor applications, it is highly advantageous to minimize the effect of the shunt resistances which are induced mainly during the processing of the photovoltaic modules.
Series connection of thin film solar cells in the module is usually accomplished by patterning the material layers using laser scribing techniques. Especially for large area modules this technique is very effective in removing layers by ablation so as to pattern the solar cells. To this end the laser parameters have to be carefully adjusted with respect to intensity, focus size and wavelength in order to appropriately remove material of the layer under consideration without unduly affecting other material layers and to provide an appropriate pattern that allows a series connection of individual solar cells.
Appropriate laser parameters can be achieved on large area robust substrates, such as glass and metal, thereby allowing highly automated manufacturing environments to be implemented for forming solar modules with series connected solar cells with a desired size, number and shape.
Nevertheless, even with well-tuned laser scribing processes it is very difficult to avoid the generation of defects, such as metal flakes during the processing in particular of the metal back side contact, thereby contributing to shunt leakages.
Furthermore, the patterning of the structures of the various layers with a laser is limited in terms of dead areas caused by the beam size, which is typically constrained to approximately 50 to 60 μm, thereby resulting in a loss of active cell area.